The goal of this project is the development of new antineoplastic agents directed against tubulin, a protein critical for cell division. Work was continued with combretastatin congeners, a series of newly isolated natural products, more active than combretastatin itself. Several members of the series are among the most potent microtubule inhibitors yet described. The mechanism of action of 2,4-dichlorobenzyl thiocyanate was examined. The drug specifically alkylates the beta-tubulin polypeptide, probably at a critical cysteine residue. Derivatives of 6-benzyl-1, 3- benzodioxole continued to interest the laboratory because their facile synthesis permits a structure-function approach to the colchicine/podophyllotoxin binding site of tubulin. Derivatives of 5,6-diphenylpridazin-3-one, which bind to a distinct site on tubulin (i.e., no competition with other drugs), continued to be evaluated in a search for maximally active agents. Alkyl carbamates of aromatic amines continued to be examined in a search for new antitubulin agents. Colchicine analogs with unusual structural features and/or biological properties were evaluated to quantitate their interactions with tubulin.